Add getPrincipal to ServerWebExchange

ServerWebExchange now has a getPrincipal method and along with that a
ServerWebExchangeDecorator that can be used to wrap the exchange in
order to return the authenticated user.

Issue: SPR-14680

spring-web/src/main/java/org/springframework/web/server/ServerWebExchange.java

@@ -16,6 +16,7 @@
 
 package org.springframework.web.server;
 
+import java.security.Principal;
 import java.time.Instant;
 import java.util.Map;
 import java.util.Optional;
@@ -68,6 +69,11 @@ public interface ServerWebExchange {
 	 */
 	Mono<WebSession> getSession();
 
+	/**
+	 * Return the authenticated user for the request, if any.
+	 */
+	<T extends Principal> Optional<T> getPrincipal();
+
 	/**
 	 * Returns {@code true} if the one of the {@code checkNotModified} methods
 	 * in this contract were used and they returned true.

spring-web/src/main/java/org/springframework/web/server/ServerWebExchangeDecorator.java

@@ -0,0 +1,109 @@
+/*
+ * Copyright 2002-2016 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.web.server;
+
+import java.security.Principal;
+import java.time.Instant;
+import java.util.Map;
+import java.util.Optional;
+
+import reactor.core.publisher.Mono;
+
+import org.springframework.http.server.reactive.ServerHttpRequest;
+import org.springframework.http.server.reactive.ServerHttpResponse;
+import org.springframework.util.Assert;
+
+/**
+ * Wraps another {@link ServerWebExchange} and delegates all methods to it.
+ * Sub-classes can override specific methods, e.g. {@link #getPrincipal()} to
+ * return the authenticated user for the request.
+ *
+ * @author Rossen Stoyanchev
+ */
+public class ServerWebExchangeDecorator implements ServerWebExchange {
+
+	private final ServerWebExchange delegate;
+
+
+	public ServerWebExchangeDecorator(ServerWebExchange delegate) {
+		Assert.notNull(delegate, "'delegate' is required.");
+		this.delegate = delegate;
+	}
+
+
+	public ServerWebExchange getDelegate() {
+		return this.delegate;
+	}
+
+	// ServerWebExchange delegation methods...
+
+	@Override
+	public ServerHttpRequest getRequest() {
+		return this.getDelegate().getRequest();
+	}
+
+	@Override
+	public ServerHttpResponse getResponse() {
+		return this.getDelegate().getResponse();
+	}
+
+	@Override
+	public Map<String, Object> getAttributes() {
+		return this.getDelegate().getAttributes();
+	}
+
+	@Override
+	public <T> Optional<T> getAttribute(String name) {
+		return this.getDelegate().getAttribute(name);
+	}
+
+	@Override
+	public Mono<WebSession> getSession() {
+		return this.getDelegate().getSession();
+	}
+
+	@Override
+	public <T extends Principal> Optional<T> getPrincipal() {
+		return this.getDelegate().getPrincipal();
+	}
+
+	@Override
+	public boolean isNotModified() {
+		return this.getDelegate().isNotModified();
+	}
+
+	@Override
+	public boolean checkNotModified(Instant lastModified) {
+		return this.getDelegate().checkNotModified(lastModified);
+	}
+
+	@Override
+	public boolean checkNotModified(String etag) {
+		return this.getDelegate().checkNotModified(etag);
+	}
+
+	@Override
+	public boolean checkNotModified(String etag, Instant lastModified) {
+		return this.getDelegate().checkNotModified(etag, lastModified);
+	}
+
+
+	@Override
+	public String toString() {
+		return getClass().getSimpleName() + " [delegate=" + getDelegate() + "]";
+	}
+
+}

spring-web/src/main/java/org/springframework/web/server/adapter/DefaultServerWebExchange.java

@@ -16,6 +16,7 @@
 
 package org.springframework.web.server.adapter;
 
+import java.security.Principal;
 import java.time.Instant;
 import java.time.temporal.ChronoUnit;
 import java.util.Arrays;
@@ -104,6 +105,11 @@ public class DefaultServerWebExchange implements ServerWebExchange {
 		return this.sessionMono;
 	}
 
+	@Override
+	public <T extends Principal> Optional<T> getPrincipal() {
+		return Optional.empty();
+	}
+
 	@Override
 	public boolean isNotModified() {
 		return this.notModified;